Conventionally, in an HEV, the voltage, current, temperature and the like of a secondary battery are detected, and the state of charge (hereinafter, abbreviated as an “SOC”) of the secondary battery is estimated by computation, whereby an SOC is controlled so as to optimize the fuel consumption efficiency of a vehicle. In order to control the SOC exactly, it is necessary to estimate exactly the SOC of the secondary battery being charged/discharged.
Conventionally, the following methods have been known for estimating SOC from a battery voltage. First, several sets of data for voltage V and a charging/discharging current I are acquired and stored in a predetermined period of time, from which a primary approximate line (a voltage (V)-current (I) approximate line) is calculated, and a V section of the V-I approximate line is calculated as a battery voltage (Voc) (no-load voltage). In addition, an accumulated value ∫I of the current I is calculated, and a polarization voltage Vp of the battery is obtained from a function including temperature T, battery voltage Voc, and current accumulated value ∫I, as variables. Electromotive force E is obtained by subtracting the polarization voltage Vp from the battery voltage Voc. Next, by referring to a previously-provided electromotive force—SOC characteristic, SOC is estimated from the thus obtained electromotive force E.
A secondary battery to be mounted on HEV or the like is configured as a battery pack formed by combining plural battery blocks formed by connecting plural cells or unit cells in series. For finding SOC at every battery block, typically the SOC calculation is performed at each battery block.
Therefore, the accuracy in SOC calculation in this case relies directly on the accuracy in measurement of the battery voltage. Accuracy in measurement of the battery voltage will be influenced by random errors, offset errors, aging errors or the like.
However, the above-mentioned conventional methods of estimating SOC from a battery voltage have problems as mentioned below.
While the voltage random error imposes fewer influences on the SOC estimation errors between the battery blocks, the voltage offset error occurs easily in the voltage between battery blocks whose voltage measurement systems are different from each other. When using a highly-accurate voltage sensor or a voltage detection circuit system for securing voltage offset error of not more than several tens of milli-volts, the cost will be raised. For this reason, a low-cost product with poor accuracy is used reluctantly in development of an inexpensive product. This will increase the estimation error of SOC between electrode blocks.